Field of the Invention
The present invention relates to a system which supplies gas enriched with nitrogen more than air to an aircraft fuel tank.
Description of the Related Art
Since an aircraft fuel tank is filled with vaporized fuel during a flight, it is necessary to prevent explosion of the fuel tank when, for example, the fuel tank is struck by lightning or a short occurs in the wiring. Thus, there has been proposed an explosion-proof system which supplies nitrogen enriched air (referred to as NEA below) having a higher nitrogen concentration and a lower oxygen concentration than air to the fuel tank.
While the air has an oxygen concentration of about 21%, the oxygen concentration of the NEA is set to, for example, 12% or less. To produce the NEA, an air separation module (referred to as ASM below) that uses a permselective membrane having different permeability coefficients for oxygen molecules and nitrogen molecules is employed. Bleed air from a flight engine is used as a supply source of air to be supplied to the ASM.
In the explosion-proof system in which the bleed air is used as the supply source of the NEA, it is necessary to increase the amount of bleed air from the engine so as to increase the amount of NEA to be supplied to the fuel tank. In this case, the fuel consumption of the engine is deteriorated. When the supply of the bleed air is increased, the oxygen concentration tends to become higher (the nitrogen concentration tends to become lower) in relation to the separation performance of the air separation module. For example, when the supply is increased to more than that by which an oxygen concentration of 10% is obtained, the oxygen concentration may be increased to 12% even by using the same air separation module.
U.S. Pat. No. 6,547,188 proposes a process for supplying NEA to an aircraft fuel tank.
In U.S. Pat. No. 6,547,188, the flight phase is divided into a phase from takeoff until entering a descent phase for landing through an ascent phase and a cruising phase (referred to as first phase below), and the descent phase (referred to as second phase below).
In U.S. Pat. No. 6,547,188, a relatively small amount of NEA is supplied in the first phase, and a relatively large amount of NEA is supplied in the second phase. It is described in U.S. Pat. No. 6,547,188 that the supply of the NEA to the fuel tank is increased in the second phase so as to compensate for an increase in the atmospheric pressure since the altitude is lowered in the second phase.
In U.S. Pat. No. 6,547,188, the concentration of the inert gas (typically, nitrogen gas) is set to a relatively high level of, for example, 98% in the first phase, and to a relatively low level of, for example, 86 to 95% in the second phase. It is described therein that the nitrogen concentration can be reduced in the second phase since the fuel tank contains a large amount of nitrogen gas in the first phase.
The aircraft fuel tank normally includes a tank (first fuel tank) provided across a fuselage portion and a pair of main wings, and a tank (second fuel tank) provided closer to the tip of each of the main wings than the first fuel tank. The NEA needs to be supplied to both the first fuel tank and the second fuel tank. However, no conventional art including U.S. Pat. No. 6,547,188 proposes a method for supplying the NEA to both of the first and second tanks. Aircraft is supplied with fuel for the next flight after landing. It is necessary to set the concentration of oxygen within each of the tanks to a prescribed value (e.g., 10% or less) before fueling. It is easily inferred that the oxygen concentration can be set to the prescribed value by supplying the NEA into each of the tanks. However, it takes time and labor to supply the NEA so as to set the oxygen concentration within each of the tanks to the prescribed value.
Thus, an object of the present invention is to provide an NEA supply system which easily maintains low the concentration of oxygen within a tank before fueling.